Rule of three - direct proportion

The rule of three is a method to solve proportions.
If you have three numbers: a, b, c, such that, ( a / b = c / x), (i.e., a: b :: c: x )
you can calculate the unknown value (x).Direct Proportion
If, a change in a quantity causes a change in unknown quantity, in the same proportion, the amounts are in direct proportion.Example
If, 4kg of a product cost $36.00, how much would the cost of 18kg?
Problem formulation.
4 kg - cost $36.00
18 kg - cost x
Full text: Rule of three - direct proportion.
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